1. Field of the Invention
The present invention relates generally to modified carbon molecular sieve (modified CMS) membranes, and more particularly to asymmetric modified CMS hollow fiber membranes.
2. Description of the Related Art
Carbon molecular sieve (CMS) membranes have shown great potential for the separation of gases, such as for the removal of carbon dioxide (CO2) and other acid gases from natural gas streams. Asymmetric CMS hollow fiber membranes are preferred for large scale, high pressure applications.
Asymmetric hollow fiber membranes have the potential to provide the high fluxes required for productive separation due to the reduction of the separating layer to a thin integral skin on the outer surface of the membrane. The asymmetric hollow morphology, i.e. a thin integral skin supported by a porous base layer or substructure, provides the fibers with strength and flexibility, making them able to withstand large transmembrane driving force pressure differences. Additionally, asymmetric hollow fiber membranes provide a high surface area to volume ratio.
Asymmetric CMS hollow fiber membranes comprise a thin and dense skin layer supported by a porous substructure. Asymmetric polymeric hollow fibers, or precursor fibers, are conventionally formed via a dry-jet/wet-quench spinning process, also known as a dry/wet phase separation process or a dry/wet spinning process. The precursor fibers are then pyrolyzed at a temperature above the glass transition temperature of the polymer to prepare asymmetric CMS hollow fiber membranes.
The polymer solution used for spinning of an asymmetric hollow fiber is referred to as dope. During spinning, the dope surrounds an interior fluid, which is known as the bore fluid. The dope and bore fluid are coextruded through a spinneret into an air gap during the “dry-jet” step. The spun fiber is then immersed into an aqueous quench bath in the “wet-quench” step, which causes a wet phase separation process to occur. After the phase separation occurs, the fibers are collected by a take-up drum and subjected to a solvent exchange process.
The solvent exchange process is an extremely important step in the membrane fabrication process. If the porous precursor fibers contain water at the time they are subjected to high temperatures, for instance during drying or pyrolysis, removal of the water causes significant changes to the structure and properties of the fiber and of the resulting CMS membrane. The high capillary forces associated with removal of water within the small radii of the pores close to the skin cause a densification of the structure in this region, which results in a less permeable membrane. To prevent this, the solvent exchange process replaces the water that is present in the porous substructure of the precursor fiber with a fluid having a lower surface tension.
A conventional solvent exchange process involves two or more steps, with each step using a different solvent. The first step or series of steps involves contacting the precursor fiber with one or more solvents that are effective to remove the water in the membrane. This generally involves the use of one or more water-miscible alcohols that are sufficiently inert to the polymer. The aliphatic alcohols with 1-3 carbon atoms, i.e. methanol, ethanol, propanol, isopropanol, and combinations of the above, are particularly effective as a first solvent. The second step or series of steps involves contacting the fiber with one or more solvents that are effective to replace the first solvent with one or more volatile organic compounds having a low surface tension. Among the organic compounds that are useful as a second solvent are the C5 or greater linear or branched-chain aliphatic alkanes.
The solvent exchange process typically involves soaking the precursor fibers in a first solvent for a first effective time, which can range up to a number of days, and then soaking the precursor fibers in a second solvent for a second effective time, which can also range up to a number of days. Where the precursor fibers are produced continuously, such as in a commercial capacity, a long precursor fiber may be continuously pulled through a series of contacting vessels, where it is contacted with each of the solvents. The solvent exchange process is generally performed at room temperature.
The precursor fibers are then dried by heating to temperature above the boiling point of the final solvent used in the solvent exchange process and subjected to pyrolysis in order to form asymmetric CMS hollow fiber membranes.
The choice of polymer precursor, the formation and treatment of the precursor fiber prior to pyrolysis, and the conditions of the pyrolysis all influence the performance properties of an asymmetric CMS hollow fiber membrane.
Important properties of asymmetric CMS hollow fiber membranes include permeance and selectivity. Permeance measures the pressure-normalized flux of a given compound while selectivity measures the ability of one gas to permeate through the membrane versus a different gas. These properties, and the methods by which asymmetric CMS hollow fiber membranes may be tested to determine these properties, are described in more detail in, for example, U.S. Pat. Nos. 6,565,631 and 8,486,179, the contents of both of which are hereby incorporated by reference.
Though asymmetric CMS hollow fiber membranes exhibit encouraging selectivities, they exhibit lower permeance after pyrolysis than would be expected based on the permeability increase in corresponding dense films before and after pyrolysis of the same precursor polymer. The lower than expected permeance is thought to be caused, at least in part, by a phenomenon known as substructure morphology collapse.
As described in U.S. patent application Ser. No. 13/666,370, the contents of which are hereby incorporated by reference, substructure morphology collapse occurs when intensive heat-treatment during pyrolysis relaxes the polymer chains, causing their segments to move closer to one another and collapsing the pores in the substructure. This substructure morphology collapse results in an increased actual membrane separation thickness, and thus a drop in permeance. Because of the permeance drop, the advantage of having a high transport flux in an asymmetric fiber is lost significantly.
In U.S. patent application Ser. No. 13/666,370, Bhuwania et al. described a method for treating precursor fibers in order to limit the substructure collapse that occurs during pyrolysis. Bhuwania et al. showed that by soaking the precursor fibers in a chemical modifying agent, such as vinyl trimethoxy silane (VTMS), before pyrolysis, asymmetric CMS hollow fibers having an increased permeance could be formed. Without being bound by any theory, Bhuwania et al. described that the chemical modifying agent thermally and/or physically stabilizes the precursor fiber prior to pyrolysis.
It has now surprisingly been found that by contacting a precursor fiber with a solution containing the modifying agent at a concentration of less than 100%, the permeance of the resulting asymmetric modified CMS hollow fiber membrane can be increased to a degree well beyond that which is achieved by soaking the precursor fiber in the chemical modifying agent alone, as was described in U.S. patent application Ser. No. 13/666,370, without having an adverse effect on the selectivity of the modified CMS hollow fiber membrane.